BEARTHITE, CA2AL(PO4)(2)OH - STABILITY, THERMODYNAMIC PROPERTIES AND PHASE-RELATIONS

The new phosphate bearthite, Ca2Al(PO4)(2)HO, found in high-pressure metamorphic rocks, has been synthesized from a stoichiometric mixture of gamma-Al2O3 and CaHPO4 . 2H(2)O between 4 kbar water pressure, 485 degrees C and 24.5 kbar, 800 degrees C. Its upper temperature stability limit, bearthite reversible arrow hydroxyapatite + corundum + berlinite + H2O, has been tightly reversed between 485 degrees C, 1 kbar, and 850 degrees C, 10 kbar. We extracted the following thermodynamic parameters for bearthite: H(f298)degrees = -4327.25 kJ/mol and S(298)degrees = 214.5 J/mol.K, using a new Delta(r)Cp approximation for dehydration reactions. Additional experiments on the reaction trolleite reversible arrow berlinite + corundum + H2O yield new thermodynamic data for trolleite, Al-4(PO4)3(OH)3 (H(f298)degrees = -6530 kJ/mol, S(298)degrees = 252.7 J/mol.K). We derived the phase relations in the CaO-P2O5-Al2O3-H2O system considering the phosphates bearthite, trolleite, berlinite, augelite, and hydroxyapatite. Extension to the SiO2-bearing system barely affects the stability field of bearthite but reveals a chemical control on bearthite occurrence. The stability of the assemblages hydroxyapatite + aluminium silicate + (quartz or corundum), with respect to bearthite + Ca-Al-silicate, restricts bearthite to very Ca-poor (Dora-Maira massif) or P-rich rocks (Monte-Rosa massif; Hohe Tauern; Vastana, Sweden). The coexistence of bearthite with a Ca-Al-silicate (lawsonite) may be possible only for pressures exceeding ca. 15 kbar under unusually low gradients of 5 degrees C/km. Bearthite is otherwise not a high-pressure index mineral but provides thermal constraints in relatively high-grade phosphate-bearing deposits or metasediments. Its stability under mantle P-T conditions makes bearthite a sink for Sr and REE in subducted Ca-poor rocks.